Add ui tests for test harness item walk and proc macro input gating

[akintewe]

These add coverage for two compiler paths that real crates hit constantly but the test suite never exercised. Found by diffing coverage from compiling crates.io crates against the test suite's coverage, then confirmed by putting a panic in the functions and compiling the crates.

These add coverage for two compiler paths that real crates hit constantly but the test suite never exercised. Found by diffing coverage from compiling crates.io crates against the test suite's coverage, then confirmed by putting a panic in the functions and compiling the crates.

Functions covered that the suite previously didn't hit:

• <InnerItemLinter as Visitor>::visit_where_predicate, visit_where_predicate_kind, visit_qself, visit_pat_field, visit_anon_const
• <EntryPointCleaner as MutVisitor>::visit_where_predicate, flat_map_where_predicate, visit_where_predicate_kind, visit_qself, visit_pat_field, flat_map_pat_field
• <GateProcMacroInput as Visitor>::visit_macro_def

For the test harness case this adds a harness revision to tests/ui/unpretty/exhaustive.rs that compiles it with --test, so the harness expansion visitors walk the complete AST instead of a hand-picked one. -Zunpretty=expanded stops before the analyses that make the hir revision fail.

The second test covers GateProcMacroInput::visit_macro_def, which only runs when an attribute proc macro is applied to an item containing a macro_rules! definition.

r? jackh726

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